It is common practice in the field of semiconductor and optics to polish glass substrates with an abrasive containing rare earth elements composed mainly of cerium oxide and additionally of yttrium oxide and neodymium oxide in small amounts. Unfortunately, Japan is limited in the yield of minerals containing rare earth elements for abrasive and she relies mostly on import for their supply. In view of the current international situation, the future uncertainly of import, and the limited reserves, there exists a strong desire to recover and recycle rare earth elements after their use.
Under these circumstances, researches are being conducted on the recovery of rare earth elements, particularly the method for recovery of cerium oxide. For example, JP-A H11-90825 (Patent Document 1) has disclosed a method for recovering cerium oxide from abrasive waste by the steps of removing impurities by treatment with an alkali metal hydroxide, forming a cake mainly containing cerium oxide with the help of a precipitant, and drying and screening.
Furthermore, JP-A 2002-28662 (Patent Document 2) has disclosed another method including the steps of adding a weak alkaline aqueous solution into abrasive waste used for polishing glass substrates, controlling the zeta potential, and centrifuging the resulting liquid to separate and recover cerium oxide.
In addition, JP-A 2007-276055 (Patent Document 3) has disclosed still another method which employs acid treatment in place of alkali treatment for recovery of cerium oxide from abrasive waste, unlike the above-mentioned method.